U.S. Pat. No. 4,524,550 discloses and claims an apparatus for removing flash from molded plastic and elastomeric articles and paint or coatings from various articles by contacting these with a blast media entrained in a cold carrier gas. The process operates by using the cold carrier gas, usually at cryogenic temperatures to embrittle the flash or coating on the articles. The embrittled flash or coating being removed by the impact of the entrained solid particles which are in the form of shot, pellets or irregular shaped particles. In one particular process, generally referred to as Cryogenic Coating Removal (CCR), hangers used on a paint line in a manufacturing operation eventually become coated with many layers of paint which have to removed. By placing these hangers in a device such as shown in the '550 patent and having the hanger contacted by particulate material (e.g., polycarbonate plastic) entrained in a nitrogen gas maintained at a temperature of between -150.degree. F. to -225.degree. F. (-101.degree. C. to -143.degree. C.) the layers of paint can be embrittled and removed.
In the device of the '550 patent the blasting particles and the coating or flash are collected in the bottom of the device, cleaned and recycled. In the apparatus of the '550 patent the carrier gas is normally vented through a tall stack. The use of a tall stack provides a means for removing particles entrained in the cold carrier gas as it exits the insulated chamber because of the velocity change as the exhaust gas ascends the chimney.
The problem with exhausting the cold gas is that it leads to an inefficient operation because a great deal of refrigeration value in the gas is lost in the exhaust. In addition, the media and particles that enter the exhaust system must be cleaned from that system regularly. Furthermore, dust or fine particles generated from the process collects on the media and can be discharged into the atmosphere since the dust particles are not heavy enough to fall back into the stack as the cold carrier gas exits the exhaust system. Another problem with using the exhaust only system is that in order to open the door to the blast treating chamber (e.g., at the end of a cleaning cycle) the exhaust system had to be turned off and when the door was open the area around the chamber door was usually bathed in dust which escaped from the chamber. Attempts to ameliorate this condition centered around putting a small collector in the exhaust system which was allowed to run for 30 to 40 seconds before the door was open to attempt to eliminate the dust.
Attempts to solve the problem centered around the use of screens in the exhaust opening. However, these have been ineffective because screens which would stop the media tend to become easily clogged and must be cleaned on a regular basis.
A cyclone separator wherein the gases are spun and then separated until the heavy particles fall to the bottom to be collected and manually returned to the blast treating chamber has been used. However, the cyclone devices must be drained of media and dust and because of its placement can be marginally effective and used only on waste gases which are then exhausted to the atmosphere. The materials reclaimed from the cyclone must be screened to remove the fines and the reusable media manually returned to the blast treating chamber. Additionally some of the media and the (fines) dust can escape through the cyclone. Another problem with cyclone separators is that by their action they utilize the inertial energy of the entrained media particles reinforced by the gas velocity to separate the media particles from the exhaust gas stream. Cyclones are bulky units and in order to work well they must be designed for a particular flowrate. If the exhaust gas flowrates are reduced, insufficient velocity of the gas will allow the media particles to fall out in the connective piping and thereby block the exhaust flow. When this happens the piping must be disassembled and the entire system cleaned before it can be reused.
The complete removal of dust from the system on a continuing basis has not been attempted before. The only attempt, as set out above involved use of a dust collector prior to opening the door at the end of the blast cycle during the coast down time of the throwing wheels in the blast chamber while at the same time preventing the door from being opened. The collector was an uninsulated unit since there was no concern about the unit being subjected to cryogenic temperatures.
There have been no attempts to recycle the cold gas to recover the refrigerative capacity of the gas prior to exhausting to the atmosphere.